Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

[Dan Williams]

On Mon, May 24, 2021 at 5:40 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
>
>
> On 5/24/2021 4:42 PM, Dave Hansen wrote:
> > On 5/24/21 4:32 PM, Kuppuswamy Sathyanarayanan wrote:
> >> Functionally only DMA devices can notice a side effect from
> >> WBINVD's cache flushing.
> > This seems to be trying to make some kind of case that the only visible
> > effects from WBINVD are for DMA devices.  That's flat out wrong.  It
> > might be arguable that none of the other cases exist in a TDX guest, but
> > it doesn't excuse making such a broad statement without qualification.
>
> We're describing a few sentences down that guests run with EPT
> IgnorePAT=1, which is the qualification.
>
> >
> > Just grep in the kernel for a bunch of reasons this is wrong.
> >
> > Where did this come from?
>
> Again the logic is very simple: TDX guest code is (mostly) about
> replacing KVM code with in kernel code, so we're just doing the same as
> KVM. You cannot get any more proven than that.
>

I have no problem pointing at KVM as to why the risk is mitigated, but
I do have a problem with misrepresenting the scope of the risk.